This disclosure relates generally to welding processes and more specifically, to methods and systems to facilitate flash reduction and weld integrity in friction stir welding processes.
Friction stir welding is typically utilized for joining together work pieces of materials such as metals and plastics. A friction stir welding tool has a shoulder at its distal end and a non-consumable rotatable welding pin extending downward centrally from the shoulder. Some known friction stir welding tools include a shoulder fixed in position with respect to the pin and other known friction stir welding tools include a pin that is movable with respect to the shoulder.
In a typical friction stir weld operation, two work pieces are supported on a backing plate and are positioned so that the work piece edges where the weld is to be formed are held in direct contact to form an interface. The tool is then positioned so that the rotating pin is in contact with the work pieces at the interface. The rotation of the pin against the work piece materials produces a large amount of frictional heating in both the welding pin and the plate. This heat causes the work piece materials to soften in the vicinity of the rotating pin, which results in the formation of a weld.
In one form of friction stir welding, the tool is moved longitudinally along the interface between the two work pieces, thereby forming an elongate weld along the interface. The welding tool shoulder facilitates reducing softened, or plasticized, work piece materials from escaping upwards and away from the weld being formed. When the weld operation is completed, the welding tool is retracted.
Known drive apparatus are utilized to move, or drive, the welding tool along the weld path. One known drive apparatus includes a conventional milling machine used in metalworking. Such milling machines are particularly suited to maintain a fixed gap between the distal end of the pin tool and the surface of the backing plate. Another type of known drive apparatus is known as a “constant vertical force machine”, and it applies a downward force along the vertical axis of the pin until a predetermined force is reached. This type of “constant vertical force” machine is particularly suited to maintaining a precise contact between the friction stir welding tool shoulder and the upper surface of the materials to be welded.
The above described friction stir welding processes are sometimes referred to as continuous path retractable pin tool welding. During such welding process, parent materials are heated to a plastic or malleable state. As explained above, as the parent materials soften, such softened materials tend to migrate or draw up between an outer diameter of the tool and an inner diameter of the shoulder holding the pin. As a result, some amount of the plasticized materials between the two surfaces escapes from the weld process, and such lost material is commonly referred to as flash. The volume of material that migrates away from the weld can sometimes cause anomalies such as voids and undesirable surface indentations.
Another known friction stir welding process is commonly referred to as friction stir spot welding. During friction stir spot welding, parent materials are heated to a plastic or malleable state. As these materials soften, they also tend to migrate up between the outer diameter of the tool shoulder and the inner diameter of a clamping anvil, allowing the plasticized parent materials between the two surfaces to escape from the weld process. As with the continuous path retractable pin tool welding process, the volume of material that migrates away from the weld can sometimes cause anomalies such as voids and undesirable surface indentations.